Multi-Terminal Phase Change Devices

ABSTRACT

Phase change devices, particularly multi-terminal phase change devices, include first and second active terminals bridged together by a phase-change material whose conductivity can be modified in accordance with a control signal applied to a control electrode. Structure allows application in which an electrical connection can be created between two active terminals, with control of the connection being effected using a separate terminal or terminals Accordingly, the resistance of the heater element can be increased independently from the resistance of the path between the two active terminals, allowing use of smaller heater elements thus requiring less current to create the same amount of Joule heating per unit area. The resistance of the heating element does not impact the total resistance of the phase change device. Programming control can be placed outside of main signal path through the phase change device, reducing impact of associated capacitance and resistance of the device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to phase change devices (PCDs) used in logic andmemory applications.

2. Description of the Related Art

A two-terminal phase change device (PCD) 10 is shown in FIG. 1, andconsists of a heating element 14 connected to a control terminal B.Heating element 14 acts as both a heater and an electrically conductingnode in thermal and electrical contact with phase change material (PCM)12, which connects to another metal terminal A which is simply anelectrically conducting node connecting the PCD 10 to other circuitry(not shown). An electrical schematic of PCD 10 is shown in theright-hand side of FIG. 1, with the heating element 14 being representedas a resistance R_(heater), and the PCM 12 being represented as aresistance R_(PCM).

In operation, a programming pulse is applied to PCD 10 in such a way asto selectively create a high resistance state or a low resistance statein the PCM 12, as explained with reference to FIGS. 2 and 3A. Inparticular, phase change materials are a class of materials which canchange phase from crystalline structures to amorphous structures or backwhen under different thermal treatments, supplied in this example byheating element 14 by way of control terminal B. When phase changematerial 14 is heated above its crystallization temperature and cooleddown gradually, it tends to form a crystalline phase and exhibits lowelectrical resistance (SET). When the phase change material is heatedabove its melting temperature and cooled down abruptly, it formsamorphous phase and exhibits high electrical resistance (RESET).Essentially, the material operates as a programmable resistor with twodistinct electrical resistance values. Phase change materials maycontain atom elements in group 4, 5 and 6 such as Ge, As, Se, Te.

FIG. 3A is a graph of the temperature profile for crystalline andamorphous phase change. Ta and Tx are melting and transitiontemperatures. t1 and t2 are time control periods for amorphous andcrystallize state formation.

The two-terminal device of FIG. 1 consists of a volume of phase changematerial contacted on one end by a low resistance metal and on the otherend contacted to a higher resistance interface. In order to program thedevice an electrical current is passed through the higher resistanceinterface into the phase change material. The heat generated in the highresistance interface along with the current injected into the phasechange material causes the phase change material to change state. Theway in which the current is removed will determine the final state ofthe material. By rapidly quenching the phase change material thematerial will be left in an amorphous state. If the materialstemperature is slowly brought through the phase transition region Thematerial will be left in a crystalline state.

Phase change materials have found their applications in optical diskmemory such as CD-RW and DVD-RW based on its optical index changeproperties between crystalline and amorphous phases. In optical diskmemory applications, a laser beam is used to introduce heat into thematerials to switch between crystalline and amorphous states which havedifferent refractive index. In integrated circuit applications, electriccurrent is used to introduce joule heating into the phase change memoryto switch between crystalline and amorphous states which have differentresistance.

A problem associated with the two-terminal device structure depicted inFIG. 1 is that the heating element must be incorporated at one end ofthe device, which means that the heating element is directly in theelectrical path between the two terminals of the device which directlyaffects the electrical characteristics of the device. This causescontention between the electrical characteristics of the device duringits programming state and its read or non-programming state. In order toSet or Reset the PCD 10, the joule heating via the heating element 14 isused to transition the PCM 12 to the appropriate temperature. Thisrequires that a trade-off be made between the electrical resistance ofthe PCD 10 and the thermal resistance of the PCD due to the heatingelement 14. In typical circuit application, designers are accustomed tobeing able to adjust resistance by making the material in the conductivepath large. In this case growing the conductive path would also meanchanging the amount of joule heating per unit area that is applied tothe PCM 12 given that the programming circuitry does not change. Theconducting path through the device incorporates the heater element whichinherently needs to be of high resistance in order to heat at low enoughcurrents. This means that a substantially high series resistance isincluded in the path between the two nodes. Thus there are threevariables with strong interdependence that can not be decoupled in the 2terminal device: resistance in the on state, current, resistance in theoff state (the last to relate directly to the heating of the Phasechange material).

Another issue with two-terminal devices is that the circuitry needed toprogram the device is directly connected to one or both of the terminalsof the device. This means that the control function that determines thestate of the device is also part of the nodes that are used to read thedevice or in any other non-programming state. This also can addcapacitive loading or current paths to the device that would be seen innormal operations at the two terminals.

BRIEF SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a multi-terminal phasechange device (PCD) is provided, allowing separation of the Set/Resetcontrols from the target electrical path. One way of accomplishing thisis by constructing a device that consists of a phase change material(PCM) that connects to a conducting terminal(s) in an electrical path.Other terminal(s) used to control the Set/Reset operations throughheater element(s) are connected separately to the PCM. This structureallows an application in which an electrical connection can be createdbetween two terminals, with the control of the connection being effectedusing a separate terminal or terminals. The benefits are manifold. Theresistance of the heater element can be increased independently from theresistance of the path between the two conductive terminals. This allowsthe use of smaller heater elements thus requiring less current to createthe same amount of Joule heating per unit area. The resistance of theheating element does not impact the total resistance of the PCD. As theimprovements are done to the heater element the resistance across thetwo conducting elements of the switch is not increased. The programmingcontrol can be placed outside of the main signal path through the PCD,reducing the impact of the associated capacitance and resistance of thedevice.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Many advantages of the present invention will be apparent to thoseskilled in the art with a reading of this specification in conjunctionwith the attached drawings, wherein like reference numerals are appliedto like elements, and wherein:

FIG. 1 is schematic diagram of a two-terminal phase change device and anequivalent electrical schematic thereof;

FIG. 2 is a graph of the temperature profile for crystalline andamorphous phase change;

FIG. 3A shows IV curves of a phase change material for crystalline (lowresistance) and amorphous (high resistance) states;

FIG. 3B is a plot relating to the programming steps by which the twostates—Set and Reset—of a PCD are realized;

FIG. 4A is a diagram of a three-terminal phase change device in astacked arrangement;

FIG. 4B is a diagram of a three-terminal phase change device in alateral arrangement;

FIG. 4C is a circuit diagram modeling the devices of FIGS. 4A and 4B;

FIG. 5 is a diagrammatic illustration of the phase behavior of aself-isolating device;

FIG. 6A is a schematic diagram of a stacked arrangement of afour-terminal phase-change device having, in addition to two activeterminals, a pair of control terminals;

FIGS. 6B and 6C are schematic diagrams of stacked arrangements offour-terminal phase-change devices;

FIG. 6D is a circuit diagram modeling the devices of FIGS. 6A-6C;

FIG. 7 is a schematic diagram depicting the use of phase change devicesin a cross bar switching structure;

FIGS. 8A-8J are schematic representations of steps of a first processflow directed at fabricating a multi-terminal;

FIG. 9 is a schematic view of a four-terminal top-connectivity device;

FIG. 10 is a schematic view of a four-terminal bottom-connectivitydevice;

FIG. 11 is a layout of a four-terminal device having a mixed top/bottomconnectivity pattern;

FIG. 12 is a layout of a three-terminal device;

FIGS. 13A and 13B are layouts of a two-terminal device, incross-sectional and top plan views, respectively;

FIG. 14 is top plan view of an arrangement showing multiple PCDs used toobtain a 3:1 multiplexer device fabricated in accordance with the firstprocess flow;

FIGS. 15A-15M relate to a second process flow for fabricating phasechange devices in accordance with the invention;

FIG. 16 is top plan view of an arrangement showing multiple PCDs used toobtain a 3:1 multiplexer device fabricated in accordance with the secondprocess flow;

FIGS. 17A-17C are schematic diagrams of phase change devices having oneor more isolated control/programming nodes;

FIG. 17D is an electrical schematic of the devices of FIGS. 17A-17C;

FIG. 18A depicts stacked-typed arrangement of a multiplexer with athree-terminal device;

FIG. 18B depicts a lateral-type arrangement of a multiplexer with athree-terminal device;

FIG. 18C is a schematic diagram of the two arrangements of FIGS. 18A and18B;

FIGS. 19A and 19B depict stacked-type (FIG. 19A) and lateral-type (FIG.19B) arrangements of pass gates using a three-terminal device inaccordance with an aspect of the invention;

FIG. 19C is an electrical schematic of the devices of FIGS. 19A-19B;

FIG. 20 is a schematic diagram of a memory array in which three-terminalphase change devices are used; and

FIG. 21 is a schematic diagram of a memory array in which three-terminalphase change devices are used, wherein one of the terminals is notconnected, and the control node is used for programming and reading.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 4A shows an example of a three-terminal PCD (phase change device)400 in accordance with the invention. PCD 400 includes a pair of activeelectrically conductive terminals, 404, 406, bridged together by phasechange material (PCM) 402. Terminals 404 and 406 connect PCD 400 toother circuit devices (not shown) whose exact nature depends on theparticular application.

PCM 402 is in thermal and electrical contact with a heating element 408,which is part of a control terminal 410 through which current is appliedto the heating element to activate same. When activated in a controlledmanner as explained further below, heating element 408 induces a phasechange in PCM 402. Two important phases are a high resistance amorphousphase, which causes the PCM 402 to be effectively high resistance,thereby establishing a virtual open circuit condition between activeterminals 404 and 406, and a low resistance polycrystalline phase, whichelectrically connects the active terminals 404 and 406 to one another ina closed circuit configuration. The two conditions are designated Reset(amorphous phases) and Set (polycrystalline phase). It should also beappreciated that there are many states of resistance between the highestresistance state and the lowest resistance state that may be useful formany types of applications.

PCM 402 is selected from a class of well-known chalcogenide materials,which include those containing atom elements in group 4, 5 and 6 such asGe, As, Se, Te and behaving in the manner described—that is, asprogrammable resistors with two different electrical resistance values.The programming steps by which these two states—Set and Reset—arerealized are further described with reference to the characteristicscurves of PCM shown in FIG. 3B. First, a voltage is applied at thecontrol terminal (410), bringing the PCM 402 to what is referred to asthe snap back region, carrier avalanche generation. At this point asmall amount voltage can dramatically increase the current through thedevice. At a given current the temperature of the heating element (408)can rise to the point at which PCM 402 reaches its phase transitiontemperature. After reaching this temperature, the mechanism for coolingthe PCM 402 determines the state in which the PCM remains afterconclusion of programming. If the PCM 402 is cooled slowly, by slowlyramping the applied voltage or current down, PCM 402 will assume thecrystalline low resistance form (Set). If the temperature of PCM 402 isreduced rapidly (the voltage quickly removed), PCM 402 will retain anamorphous, high resistance form (Reset). Thus a typical programmingregime simply requires that the appropriate voltage waveform be appliedto the device 400. It will be appreciated that during programming—thatis, in a programming mode when a control signal is being applied to thecontrol terminal 410—an electrical path is established between thecontrol terminal and one of the active terminals 404 or 406, in the formof a current passing between the control terminal and one the activeterminals (in either direction), for example by way application of avoltage (positive or negative) across the control terminal and one ofthe active terminals. This electrical path activates heating element408, which presents a finite resistance, and which induces the phasechange in the PCM 402. Conversely, when not in the programming mode—thatis, when the PCD is operating in a field mode after it has beenprogrammed—the control terminal 410 is substantially electricallyinactive. It is removed or isolated from the PCD, and ideally does notdraw current or add current, or present any loading on the circuit.

In the arrangement of FIG. 4A, the device is herein referred to as astacked three-terminal PCD. Alternatively, a lateral three-terminal PCDarrangement can be provided, as shown in FIG. 4B. Lateral three-terminalPCD 430 has a different physical layout, consisting of first (414) andsecond (416) active terminals that overlie one another, and that arebridged by a PCM 412 in thermal contact with a heating element 418. Acontrol terminal 420 activates the heating element 418, causing thechange in phase of the PCM 412 in the manner described above.

The lateral arrangement behaves in an effectively identical manner tothat of the stacked arrangement, and both can be modeled by the circuitshown in FIG. 4C, which depicts a switch between nodes A and B which canbe selectively opened (high resistance, amorphous phase of PCM 412) orclosed (low resistance, polycrystalline phase of PCM 412). However, thephysical layout and fabrication processes of the two arrangements aredifferent, and different advantages may inure to each of thearrangements depending on the contemplated application. In the case ofthe lateral PCD 430, a very small heating element 418 can be realized,using atomic layer deposition, rather than using via or contactprocessing techniques which may be required for the stacked arrangementand which are limited by photolithographic and etch techniques. This mayrequire a larger cell area in the X,Y plan but the device scales betterin shrinking technologies than the stacked arrangement.

The multi-terminal device PCD can be used as a means of creating across-bar or switching fabric in a programmable device. The two methodsof creating a cross bar structure are a cross-point structure and a muxstructure. The cross-point structure simply connects two wires directlyvia some type of switch, which, in accordance with the invention, can bea multi-terminal device switch 700, four of which are shown in FIG. 7.Each wire-2-wire connection has exactly one switch associated or onecross-point of contact. The mux structure uses multiple switches or passgates in order to route one of multiple wires to one other wire, and bylayer multiple multiplexers together one can achieve a structure thatallows one ore more wire to connect to one or more other wires.

Typically the cross bar functions as a means of electrically connectingone or more nodes to one or more other nodes. The electricalcharacteristics of the cross bar are defined by the application that itis being used in. Often a very low resistance connecting state isrequired, less than 1K ohms, a high off-resistance state is desired toreduce leakage through the non-selected lines, and in the configurationof FIG. 7, the ability to control the connection with an alternatecontrol node is realized.

A multi-terminal phase change device in accordance with the invention isultimately more useful to such an application than a two terminal devicefor certain applications. It removes the control/programming element outof the direct path of the electrical connection of the two wires. Italso gives a level of electrical separation from the wires that arebeing connected in the crossbar.

In addition to the Set and Reset states, a third state can be induced inthe PCD, as explained with reference to FIG. 5. This state, hereinreferred to as self-isolation, can be realized if the Set voltage pulsediscussed above is followed by a controlled pulse that leaves portion ofthe volume (503) of PCM 502 in the amorphous, high resistance phase,while leaving the larger portion of the material in its low resistancephase. The volume of amorphous material 503 will ideally be adjacent tothe heating element 508, effectively isolating the heater node from theremainder of the device 500, which may be important for reducing leakagecurrents or loading by the programming control terminal 520 whichthereby disturb the behavior of PCD 500 or other devices (not shown)connected to terminals 504 and 506.

Another method of implementing a mutli-terminal device would be to haveone or more of the control/programming nodes isolated with some type ofbarrier material that under high fields would allow for current to beinjected into the substrate to assist the programming operation butunder normal operating conditions would not have any current or veryminimal current attributed to that node(s). This may be necessary forinstances where the heating element is so resistive that not enoughcurrent would actually be available to assist in the programming of thedevice. Such structure is illustrated in FIGS. 17A-C, and an equivalentschematic is provided in FIG. 17D. FIG. 17A relates to a stacked-type ofarrangement, while FIGS. 17B and 17C relate to a lateral-type ofarrangement.

Other devices can be realized with more than three terminals. FIG. 6Ashows a four-terminal PCD 600 having, in addition to active terminals604 (In A) and 606 (Out), a pair of control terminals 610 and 611 atleast one of which is in thermal communication, by way of a heatingelement (608), with PCM 602. Although only one of the programming nodesin FIGS. 6A-6C are shown with at heating element the, device could beconstructed with both programming nodes having a heating element. Acurrent path between the control terminals 610 and 611 is thus formed,across PCM 602. Set/Reset circuitry (not shown) is connected to controlterminal 610, and a suitable current/voltage programming regime isapplied in order to transition PCM 602 to the desired phase/resistancestate. The use of more than one programming node enhances the ability tomore completely heat the PCM and thus reach a better ratio between thehigh resistance and low resistance states. Using multiple programmingnodes also allows for the creating of a conducting path for programmingwithout having to go through any of the terminals that are used in thenormal operation. This potentially removes some of the loading onfunction wires in the device. Further, while only one current sink(terminal 611) is shown, the invention is not so limited and multiplecurrent sinks can be provided to thereby achieve an optimum Set/Resetresistance ratio and improve reliability and fabrication yield. Terminal611 can also be provided with a second heating element (not shown)adjacent PCM 602 to supplement the heating element of terminal 610. Anadvantage of a second heating element opposite heating element 608 wouldbe symmetrical heating of PCM 602, on both sides of the PCM, requiringless power to achieve the desired phase change in the material and lesspower in the overall system in which multiple PCDs such as PCD 600 areused.

Four-terminal PCD 600 can have various configurations in addition to thestacked arrangement of FIG. 6A. In particular, as shown in FIGS. 6B and6C, lateral arrangements having different input and output terminals canbe realized. An equivalent electrical representation is shown in FIG.6D. The Isolate terminal is provided in order to prevent current leakagethrough the current sink. This is effected by turning the pass gate 612off (that is, open-circuiting the path to ground) when PCM (602 in FIG.6A) is being operated in the conductive (polycrystalline) state, butturning it on (short-circuiting the path to ground) when the terminal611 (in FIG. 6A) is operating as a current sink especially during theSet/Reset programming operations.

Depending on the circuitry and architecture of the overall device, itmay be better to use PCDs having a stacked structure or a lateralstructure or both. The placement of the heating element will also dependon the circuitry and the architecture. In some circumstances it isdesired to bring the heating element up to layers above the device inorder to get the most efficient use of wires in the upper layers of theintegrated circuit. There may be a desire to bring the programming nodeto the layers below quickly, such as transistors that would be used todrive the current into the heater element.

The multi-terminal device can take on the function of a CMOS pass gate,multiplexer, OR gate, AND gate, and so forth. FIG. 18A depictsstacked-typed arrangement of a multiplexer with a three-terminal device.FIG. 18B depicts a lateral-type arrangement. A schematic illustration ofthese two arrangements is provided in FIG. 18C. FIGS. 19A and 19B depictstacked-type (FIG. 19A) and lateral-type (FIG. 19B) arrangements ofpassgates using a three-terminal device in accordance with an aspect ofthe invention.

FIG. 20 is a schematic diagram of a memory array in which three-terminalPCDs devices (3-T PCD) are used in accordance with the invention. Avariation on this arrangement is shown in FIG. 21, in which one of theterminals of the 3-T PCDs is not connected, and the control node is usedfor programming and reading.

There are various ways to implement the switch fabric that incorporatesa separate set of terminals programming the device. It can also beconceived that the input and the output to the device can be created asa plurality of nodes, such that one or more wires can be connected tomore than one output.

Phase change devices (PCDs) can be used as memory elements, logicswitches or programmable resistors. Each flavor of device may require adifferent number of terminals and may need a different connectivitystrategy, based on the circuit application. For standard integratedcircuits all these flavors of devices may be used on the same chip.Therefore, in order to reduce manufacturing costs, it becomes importantto develop a manufacturing flow that allows building these devices withthe same process steps. The type of device is controlled primarily bylayout and programming conditions. PCD fabrication process flows inaccordance with the invention may offer one or more of the followingadvantages:

1) Allowing fabrication of a multi-terminal phase change device2) Providing integration above metal 1 and at any metal level (Metalx)3) Symmetrical placement of heating elements on both side of the PCMmaterial for more efficient Joule heating and better thermal isolation4) Allows building of devices having different configurations (twoterminal or multi-terminal) on the same chip using the same processsteps5) Allows flexible connection to the terminals as required for a moreefficient connectivity to the rest of the circuitry6) Only one or two extra masks needed compared to prior art

FIGS. 8A-8I are schematic representations of steps of a first processflow directed at fabricating a multi-terminal PCD 100 depicted incross-section and top plan view in FIG. 8J. The process begins in Step 1with a wafer after full CMOS processing up to metalx 101, which isexemplarily any metal layer in the CMOS process. Metalx 101 issurrounded by a dielectric material 102 and is covered with an etch stoplayer 103, as shown in FIG. 8A. This is usually the case for both AlCu(Aluminum-Copper) and Cu (Copper) BEOL (back-end of the line) flows. Thecross-section in FIG. 8A is of planar surface that is typical of the CuBEOL flow. However, similar process steps can be applied to an AlCuflow.

In Step 2, a via hole 104 is patterned into the etch stop layer 103 toopen a contact to the metal below. A diffusion barrier metal 104 a suchas TaN or TiN of 250 Å is deposited and patterned to cover the via holein Step 3, as seen in FIG. 8B. W or CoWP can also be used as thediffusion barrier material. The diffusion barrier metal 104 a representsthe interface material to the bottom electrode or Input/Output pin forthe multi-terminal PCDa (for example IN A in FIGS. 6B and 6C). For moreadvanced Cu metallization schemes using diffusion barriers selectivelygrown on top of the Cu, step 2-3 in the flow described below can beskipped resulting in further simplification and reduction of processsteps.

FIG. 8C relates to Steps 4 and 5. In Step 4, a dielectric (˜1000 Å) 105of the same material typically used for ILD (Inter-Level Dielectric) isdeposited. Then, in Step 5, via holes 106 are etched in the dielectric,landing on the metal below. The via holes are filled with metal 106 aand planarized. The via holes represent the connection of the programpins to the heater elements and may be needed only when connection tometal below is preferred. Otherwise, this process step can be omitted.

In Step 6 a diffusion barrier/etch stop stack 107 is deposited, usingfor example TaN/SiN (Å200 Å/250 Å), and patterned to define the heaterregion, or W or CoWP. This is illustrated in FIG. 8D. Next, in Step 7(FIG. 8E), an additional ILD dielectric layer 108 is deposited (˜1000 Å)on top of the heater stack 107. In Step 8, a hole 109 is etched throughthe heater stack into the dielectric as shown in FIG. 8F).

In Step 9, phase change material (PCM) 109 a is deposited into hole 109,as illustrated in FIG. 8G. In order to obtain a good fill, the hole 109is designed to have a low aspect ratio. This is achieved by limiting thehole height. The deposited material is then planarized.

A diffusion barrier/etch stop stack 110 (TiN or TaN/SiN (˜200 Å/250 Å)or W or CoWP) is deposited, in Step 10, to allow sealing of the PCM andprevent material inter-diffusion at the interface with the topelectrode. Step 10 is illustrated in FIG. 8H.

In Step 11, ILD oxide 111 is deposited and planarized, as shown in FIG.8I. At this point standard BEOL processing for defining vias 112 andmetal 113 can be resumed.

It will be appreciated that the etch stop layer on top of the heater andPCM seal layers help prevent over-etching and punching through thediffusion barrier film while etching the standard Mx to Mx+1 vias. Thisshould help improve via resistance.

The device of FIG. 8J is configured to have top and bottom connectivity.It will be appreciated that a similar process flow can be used toproduce a four-terminal top-connectivity device 100 _(top) illustratedin FIG. 9, and a four-terminal bottom-connectivity device 100 _(bot)illustrated in FIG. 10. FIG. 11 illustrates the cross-sectional view ofa four-terminal device 100 _(four) having a mixed top/bottomconnectivity pattern fabricated using a similar process flow, Device 100_(four) has a device floorplan that is <20F2. This is three to fourtimes smaller than the area of prior art memory cells using two terminalphase change devices. FIG. 12 illustrates the cross-sectional view of athree-terminal device 100 _(three). FIG. 13A illustrates across-sectional view of a two-terminal device using a similar processflow. FIG. 13B is a multi-terminal layout view of a device fabricated inaccordance with the first process flow. FIG. 14 illustrates the layoutof a 3:1 multiplexer device fabricated in accordance with the firstprocess flow.

FIGS. 15A-15M relate to a second process flow for fabricating phasechange devices in accordance with the invention. Beginning with Step 1,described with reference to FIG. 15A, the process again starts with awafer after full CMOS processing up to Metalx 201 surrounded bydielectric 202. Metalx 201 is covered with an etch stop layer 203. Thisis usually the case for both AlCu and Cu BEOL flows. The cross-sectionshows a planar surface that is typical of the Cu BEOL flow. However, thesame process steps can be applied also to the AlCu flow.

In Step 2 (FIG. 15B), an oxide layer 204 of the same kind used forinter-level dielectric (ILD) is deposited (1000 Å), and a heater hole205 is etched to land on the metal below. Sidewall spacer techniques(not shown) can be used to reduce the hole size below the minimumlithography feature size. This step is an option if the heater-to-PCMinterface must be reduced to reduce the programming current.

Step 3 is illustrated in FIG. 15C. In Step 3, the heater hole 205 isfilled with diffusion barrier material 205 a using a CVD (chemical vapordeposition) technique to achieve conformal step coverage and good fillof the hole 205.

In Step 4, via holes 206 a are etched to land on Metalx 201. Via holes206 a are filled with conductive materials 206 using well knowntechniques, as shown in FIG. 15D. Excess diffusion barrier left from thedeposition in Step 3 is removed during Cu CMP (chemical-mechanicalpolishing). This is only required when bottom-connectivity is desired.

Next, in Step 5, a diffusion barrier metal cap 207, such as TaN or TiNof 250 Å is deposited and patterned to cover the via materials 206 a, asseen in FIG. 15E. W or CoWP can also be used. This metal structurerepresents the bottom electrode or input/output pin for themulti-terminal device. The diffusion barrier metal caps help preventmaterial inter-diffusion between PCM and via metals. These steps can beomitted if connectivity at Metalx is not desired.

In Step 6 (FIG. 15F), a dielectric layer 204 a is deposited (1000 Å) ofthe same material as the ILD layer and PCM layer trenches 208 arepatterned and etched.

Then, in Step? (FIG. 15G), a PCM layer 208 a is deposited and planarizedusing CMP (chemical-mechanical polishing).

Step 8, illustrated in FIG. 15H, entails deposition and patterning of aPCM interface barrier stack 209 (TiN 200 Å/SiN 250 Å). This step is onlyrequired for connectivity from the top.

An ILD layer 209 a is then deposited, and a heater hole 210 is thenetched, in Step 9, to land on the PCM film 208 a, as seen in FIG. 15I.Sidewall spacer techniques (not shown) can be used to reduce the holesize below the minimum lithography feature size. This is an option ifheater-to-PCM interface must be reduced to reduce the programmingcurrent. The heater hole 210 is filled with diffusion barrier material210 a using a CVD technique (500 Å TiN) to achieve conformal stepcoverage and good fill of the hole, as seen in FIG. 15J. The excess filmfrom the surface can be removed by CMP.

In Step 10, a heater top electrode barrier cap (TiN 250 Å) 211 isdeposited and patterned, as shown in FIG. 15K. Then, in Step 11, ILDoxide is deposited and planarized. At this point the standard BEOLprocessing for defining vias 212 (FIG. 15L) and metal 213 can beresumed, to yield the dual damascene structure 200 shown in FIG. 15L(cross-section) and FIG. 15M (top plan view). It will be appreciatedthat the etch stop layer on top of the GST seal layers help preventover-etching and punching through the diffusion barrier film whileetching the standard Metalx to Metalx+1 vias. Dielectric layerthicknesses can be varied based on the desired total thickness for theILD and IMD layers.

As in the first process flow, by using this flow it is possible togenerate devices with connectivity at different metal levels and deviceswith variable number of terminals, including the more commontwo-terminal device. This can be accomplished by simple modification ofthe layout. The device floor plan is <20F2. This is three to four timessmaller than the area of prior art memory cells using two terminalsphase change devices.

The devices can be easily stacked to obtain any desired mux size (100b), as shown in FIG. 16 (3:1 mux).

The above are exemplary modes of carrying out the invention and are notintended to be limiting. It will be apparent to those of ordinary skillin the art that modifications thereto can be made without departure fromthe spirit and scope of the invention as set forth in the followingclaims.

1. A multi-terminal phase-change device (“PCD”) comprising: first andsecond active terminals for establishing electrical connections capableof passing a logic signal; a phase-change material (PCM) disposedbetween said first and second active terminals; a heating elementcoupled to said PCM and configured to provide Joule heating to modifyelectrical conductivity of said PCM; and a control terminal coupled tosaid heating element and configured to provide a programming current toactivate said heating element.
 2. The device of claim 1, wherein saidlogic signal is configured to travel from said first active terminal tosaid second active terminal via said PCM.
 3. The device of claim 1,wherein said PCM includes one or more of Group IV or Group V elements.4. The device of claim 1, wherein said first and second activeterminals, PCM, and control terminal are arranged in a stackedconfiguration.
 5. The device of claim 1, wherein said first and secondactive terminals are respectively connected to first and secondcross-bar wires of a switch fabric in a programmable logic device. 6.The device of claim 5, wherein said first and second active terminalsare configured to selectively establish an electrical connection betweensaid first and second cross-bar wires based on a control signal appliedto said control terminal.
 7. The device of claim 1, wherein said controlterminal is configured to program said PCM to one of a substantiallyelectrically conductive state and a substantially electricallynonconductive state.
 8. The device of claim 1, wherein said controlterminal programs said PCM to a self-isolating state for reducingleakage current.
 9. The device of claim 1, wherein said first and secondactive terminals, PCM, and control terminal are configured as componentsof a logic switch.
 10. The device of claim 1, wherein said first andsecond active terminals, PCM, and control terminal are configured ascomponents of a CMOS pass gate.
 11. The device of claim 1, wherein saidfirst and second active terminals, PCM, and control terminal areconfigured as components of a multiplexer.
 12. A phase-change devicecomprising: a first active terminal having a first end and a second endand configured to receive and transmit a current representing a logicsignal; a phase-change material (PCM) bridge having a first end and asecond end, wherein said first end of PCM is coupled to said first endof first active terminal; a second active terminal having a first endand a second end, wherein said first end of second active terminal iscoupled to said second end of PCM; a heating element coupled to said PCMbridge and configured to program electrical conductivity of said PCMbridge via thermal effect; and a control terminal coupled to saidheating element configured to provide a programming current to activatesaid heating element.
 13. The device of claim 12, wherein said firstactive terminal is configured to transmit said logic signal to saidsecond active terminal when said PCM bridge is programmed to anelectrical conductivity state.
 14. The device of claim 12, wherein saidfirst active terminal and said second active terminal are logicallydisconnected when said PCM bridge is programmed to an electricalnon-conductivity state.
 15. The device of claim 12, wherein said firstand second active terminals, PCM, and control terminal are configured tobe one of an OR gate, inverter, and an AND gate.
 16. The device of claim12, wherein said control terminal is configured to program said PCM toone of a substantially electrically conductive state and a substantiallyelectrically nonconductive state.
 17. A programmable point cross barcomprising: a plurality of input terminals for receiving a plurality ofinput signals; a plurality of output terminals configured to output aplurality of output signals; a plurality of control terminals configuredto provide programming signals; and a plurality of three-terminalbridges having phase-change material (PCM) for providing programmableconnectivity based on programming signals, wherein each of saidplurality of three-terminal bridges includes, a first active terminalcoupled to one of the plurality of input terminals, a second activeterminal coupled to one of the plurality of output terminals, a thirdprogramming terminal coupled to one of the plurality of controlterminals for providing a programming signal to program electricalconnectivity of PCM in one of the plurality of three-terminal bridgesvia a heating element.
 18. The cross bar of claim 17, wherein saidheating element is configured to program electrical conductivity of saidPCM in response to said programming signal.
 19. The cross bar of claim18, wherein said heating element is able to generate Joule heating inresponse to said programming signal to modify electrical conductivity ofsaid PCM.
 20. The cross bar of claim 19, wherein said programming signaltravels from said third programming terminal to said second activeterminal passing through said heating element and at least a portion ofsaid PCM for varying temperature of said PCM.